Leaf- or bump-type foil air bearings are self-acting, compliant surface bearings. When in operation, they support load on an air film. Benefits of foil air bearings include reduced cost, higher speed, lower weight, higher efficiency and greater accommodation for misalignment and distortion. During start-up and shutdown, however, unlubricated dry high-speed sliding contact occurs between e.g. the shaft and foil surfaces, causing wear, until the air film was formed. The lift-off speeds depend from design and diameter, but range typically from 8 m/s to 15 m/s. A wear resistant coating is therefore required for durable, long-term operation. They accumulate up to 100,000 starts and stops.
The foils thicknesses are below 200 μm and a wear loss of 25% of this thickness is only tolerable otherwise the damping characteristics change. Additional, the use of coated shafts in air foil bearings is considered to be necessary, since special shaft super-alloys like, INX-750, René41, INC909 or MA956 tend at 800° C. to adhesive (Literature 1) material transfer, when sliding unlubricated against INX-750.
The substitution of chromium carbide (U.S. Pat. No. 5,034,187) in the NASA PS200-series self-lubricating coating based on 30%-70% chromium carbide, 5%-20% soft and nobles (Ag) metals, 5%-20% fluorides and 20%-60% metallic binder by chromium oxides leading to the NASA PS300 series (U.S. Pat. No. 5,866,518) generic composed of 60-80% Cr2O3, fluoride of gr. I and II, Ag, Au, Pt, Pd, Rh or Cu and NiCr binder seems to be motivated by cost and process aspects. The porosity of a sintered PM212 was 22% by volume. The NASA-series PS302 to PS321 present a binder rich (41-60 wt.-% of NiCr, ˜METCO 443) concept.
The use of solid lubricant coatings composed of 20-23% BaF2+13-15% CaF2+33-38% Cr2O3+25-33% Ag in a heat resistant Ni-binder (23, 5Ni17Co12, 5Cr0, 5Al) was first presented by S. Niizeki et al. in literature 3, S. Niizeki, T. Yoshioka, H. Mizutani, H. Toyota, T. Hashimoto, Development of solid lubricants for high temperature rolling ceramic bearings (part I): Various solid lubricants based on fluorid compounds, Japanese Journal of Tribology, Vol. 40, No. 12, 1995, S. 1277-1287.
The wear rates for the couple INX-750/PS304 at 25° C. and 500° C. in a narrow range of 1.6-2.4 10−4 mm3/Nm were measured in the trust-washer rig under P×V-values of 0.11-0.22 MPa×m/s by NASA.
The PV factor or P×V value is the product of bearing pressure and surface velocity traditionally expressed in (lb/in2)×(ft/min) or in the ISO equivalent in Pa×m/s. It characterizes the mechanical input in a sliding bearing as severity of design or the maximum mechanical input that the tribomaterials can support. The term μPV is equivalent to the heat generated in W/mm2. It describes a barrier between low wear and high wear regimes. As well as the upper value of a load-bearing materials PV product above which a material fails to function satisfactorily. High PV-values mean high load capacities of the bearing.
The world wide used wear rate or wear coefficient in [mm3/Nm] describes the wear expectation of a couple under certain operating conditions, but is not a material property. It is defined as the ratio of the wear volume divided by the acting normal force and effective sliding distance.
For definition of wear rate and PV-value, please see: ASM handbook “Friction, Lubrication and Wear Technology, volume 18, October 1992, ISBN 0-87170-380-7 or
ASTM G40-01 “Standard Terminology Relating to Wear and Erosion”.
U.S. Pat. No. 5,370,719 is directed to cutting inserts and displays results from milling test. They form open tribosystems. Since the normal load on the cutting edge is unknown, wear rates cannot be calculated using the wear scar width.
The patent application DE Patent No. 195 30 517 discloses also a monophasic coating composed of (Ti,Mo)(C,N) deposited on steel for cutting tools. In the common understanding of metallurgist is “steel” distinct from a super alloy based on nickel or cobalt.
The patent applications related to titanium carbonitride or titanium-molybdenum carbonitride cermets cover cutting tool inserts. Cutting tools form an open tribosystems, where the cutting tool interacts continuously with the working peace, which itself enters into contact mainly or only one time. The working peace consists consequently not of the cermets.
The foil bearing itself represents a closed tribosystems composed of two periodically interacting surfaces, which is distinct different from an open one.
DE Patent No. 195 48 718 discloses lubricious oxide (LO) coatings for the tribological stressed surfaces in liquid lubricated internal combustion engines. The LO can be deposited as coatings or form LO by tribooxidation on substrates, like (Ti,Mo)(C,N). The type of binder is not mentioned. The LOs aim to substitute extreme pressure (ep) and anti-wear (aw) additives as well as polymeric viscosity index improvers in liquid lubricants (engine oils). In consequence, these LOs run under mixed/boundary and/or hydrodynamic liquid lubrication and not under dry friction (unlubricated) or at high temperatures as in the foil bearing.
U.S. Pat. No. 6,020,072 and U.S. Pat. No. 6,017,592 disclose a dry running hinge joints for aerospace applications up to 1600° C., which operate obviously at low speeds (v<0.5 m/s) and are unable to form an aerodynamic film. Tribooxidation or static oxidation form soft reaction layers on specific materials, like (TiMo)(C,N), typically Magnélli-type phases as substoichiometric oxides forming distinct planar oxygen defects. The type of binder for use as thermal sprayed coating is not mentioned. The substrates are made of SiC, HfC and C-SiC composites. The first and second component forming the closed tribosystem (hinge joint) use the coatings disclosed in these patent applications. As coating, also Tin-2Cr2O2n-1 is disclosed without any data about the tribological behavior for sliding velocities above 1 m/s.
US Patent Published Application No. US2004/113520 disclose (Ti,Mo)(C,N)+8-20% Ni/Mo-binders (TM 8, 10, 20 grades of Sandvik Hard Materials) for piezoelectric high-power or vibration motors. The tribosystems in these motors perform by nature a linear, oscillating motion with strokes below 15 μm at oscillating frequencies of 20-40 kHz. The significant differences to foil bearings are the 1.000 times greater generation of frictional heat during dry sliding of foils and the fact of needing coefficient of friction greater than 0.5. The piezomotor is unable to form an aerodynamic film.
DE Patent No. 196 40 789 discloses coatings for liquid lubricated piston rings in internal combustion engines composed of 50-95% by volume (Ti,Mo)(C,N) hard phase with Ni,Co,Fe-binders, which were as example deposited using powders composed of 59.6 weight-% TiC0.7N0.3, 12.0 weight-% Mo2C and 28.4 weight-% Ni.
The results in L.-M. Berger et al “Hartmetallähnliche Schichten gegen Verschleiβ and Korrosion”, Maschinenmarkt No. 8, 1996, for abrasion resistance tests according to ASTM G65-85 or of erosion tests with an open tribosystem of cermets and hard metals can't be transferred to the air foil bearing, as no third bodies (abrasives) are welcome or abrasives will impact in bearing surfaces in a aerodynamic foil bearing and of course are not there present, since particle in the size of the air film will cause damage and block the tribosystem.
The results of Skopp, A. and M. Woydt “Ceramic and Ceramic Composite Materials with Improved Friction and Wear Properties”, Tribology Transactions, Vol. 38(2), 1995, p. 233-242 of the unlubricated tribological behaviour in air of self-mated TM10 (RPK of smooth rotating disks ˜0.025 μm) sliding couples up to 800° C. and 3.68 m/s can't be taken into consideration, as they were achieved using a monolithic (Ti,Mo)(C,N) with a binder composed of 13% by weight of nickel and 2% by weight of molybdenum.
Magnélli-type phases of (Ti,V)nO2n-1 or (W,Mo)nO3n-1 are unsuited for temperatures in air above 400-450° C. as they reoxidize to the stoichiometric composition. This can be overcomed by the use of Tin-2Cr2O2n-1, with 6≦n≦9, which is oxidatively stable up to 1000° C. and above.
The state-of-the-art for sliding materials for foil bearings, especially established by the PS30x-coatings, disclose wear rates limited to 10−5 mm3/Nm.